Under the present circumstances in which global environmental issues including the increased emission of carbon dioxide are emphasized, solar light power generation has come to receive attention again, besides the effective utilization of water power, wind force, geotherm, etc.
For solar light power generation, use is generally made of a solar cell module produced by protecting a solar cell element, such as a silicon, gallium-arsenic, or copper-indium-selenium element, with an upper transparent protective material and a lower substrate protective material and fixing the solar cell element and each protective material to each other with a resinous encapsulant to fabricate a package thereof. Since such solar cell modules can be disposed scatteringly in places where electric power is necessary, although on small scales as compared with electric-power generation by water power, wind force, etc., researches and developments are being pursued with the aim of improving performances, e.g., power generation efficiency, and reducing the cost. In addition, since the country and municipal corporations are taking a policy for helping with installation costs as an undertaking for introducing residential solar cell power generation systems, the systems are spreading gradually. However, further reductions in cost are necessary for further spread. Consequently, not only investigations for developing a solar cell element employing a new material, as a substitute for the conventional silicon, gallium-arsenic, and other elements, but also efforts to further reduce the cost of solar cell module production are being steadily continued.
With respect to requirements for the solar cell encapsulant as a constituent component of a solar cell module, the encapsulant is required to have satisfactory transparency from the standpoint of ensuring the quantity of incident solar light so as not to reduce the power generation efficiency of the solar cell. Meanwhile, solar cell modules are usually installed outdoors and hence heat up as a result of long-term exposure to solar light. From the standpoint of avoiding the trouble that the resinous encapsulant flows due to the heating-up to cause a module deformation or the like, the encapsulant must be one having heat resistance. Furthermore, thickness reductions are being advanced year by year in order to reduce the cost of materials for solar cell elements, and a encapsulant having excellent impact resistance is also desired from the standpoint of cell protection.
For example, a composition obtained by incorporating organic peroxide into an ethylene/vinyl acetate copolymer (EVA) having a high vinyl acetate content to impart a crosslinked structure thereto has conventionally been employed as encapsulant in solar cell modules from the standpoints of cost, processability, moisture resistance, etc. (see, for example, patent document 1). However, such ethylene/vinyl acetate copolymer (EVA) resins, when used over a prolonged period, suffer deteriorations or alterations, such as yellowing, cracking, and foaming, and hence decrease in moisture resistance. Use of these resins has thus resulted in a decrease in power generation amount due to the corrosion of the solar cells, etc. These deteriorations are thought to be because the EVA resins have a highly hydrolyzable ester structure and are hence prone to be affected by solar light and water.
An encapsulant for solar cell modules which includes a noncrystalline or lowly crystalline α-olefin-based copolymer having a degree of crystallinity of 40% or less has hence been proposed (see patent document 2). Patent document 2 shows an example in which organic peroxide is incorporated into a lowly crystalline ethylene/butene copolymer and a sheet is produced from the mixture using a profile extruder at a processing temperature of 100° C. However, there is a problem in that the production efficiency cannot be heightened because of the low processing temperature.
Also proposed as a encapsulant for solar cell modules is a polymeric material including a polyolefin copolymer which satisfies one or more of the following requirements: (a) a density less than about 0.90 g/cc, (b) a 2% secant modulus, as determined in accordance with ASTM D-882-02, of less than about 150 megapascals (MPa), (c) a melting point lower than about 95° C., (d) an α-olefin content of at least about 15 and less than about 50% by weight based on the weight of the polymer, (e) a Tg of less than about −35° C., and (f) an SCBDI of at least about 50 (see patent document 3).
In solar cell modules, the solar cell encapsulant tend to become thinner as the thickness of the solar cell elements is reduced. This tendency has posed a problem in that the solar cell module is prone to suffer wire breakage when an impact is given thereto from the upper or lower protective material side of the solar cell encapsulant. Although it is desired to heighten the rigidity of the encapsulant in order to mitigate the problem, the polymeric material of patent document 3 has had a problem in that heightening the rigidity thereof results in impaired crosslinking efficiency.
As described above, a resin composition for solar cell encapsulant which is excellent in terms of crosslinking property, heat resistance, transparency, and impact resistance has not been obtained with any conventional technique.